find the flux of F through part of a surface that lies above a plane

Hello,
We were given this problem for homework but I am confused about solving it...
Question: Find the flux of F = y^3i + z^2j +xk downward though the part of the surface z=4-x^2-y^2 that lies above the plane z=2x+1.
I understand how to compute a flux but where I am confused is with what I should do with the plane z=2x+1.
If anyone can help I would really appreciate it.